The unnamed middle Eocene sandstones of Scow Bay that are well exposed on the beaches of Indian and Marrowstone Islands, northwestern Washington, document a previously unknown local basement high that shed sediment south onto a subsea fan from the San Juan Islands terranes. The sandstones are lithic arenites with a variety of lithic grains, predominantly volcanic and sedimentary (including chert). Rare quartz-plagioclase plutonic and low-grade metamorphic (mainly chlorite-rich) lithic grains are also present. Quartz and plagioclase comprise most of the nonlithic grains. Rare heavy minerals and potassium feldspar are also present. The San Juan Islands terranes were the source area. Paleocurrent directions obtained from flute casts support this interpretation.

The sandstones are thin to very thick-bedded with minor shale interbeds and at least two 20 to 30-m (65 to 100-ft) thick shale beds. The sandstone beds are commonly structureless, although dish structures, poorly developed parallel lamination, load casts, and shale rip-up clasts are abundant locally. Soft sediment deformation, including slumps, is locally evident. Amalgamation of thinner sandstone beds into very thick (3-5 m, 10-16 ft) beds is common but often difficult to recognize owing to their structureless nature. The exposure along the east coast of Indian Island reveals at least 11 thinning- and fining-upward sequences. The sandstones were deposited as channel-fill sequences on the midfan region of a subsea fan. The thick shale beds were deposited between active channels.

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